Barnacle cement: a polymerization model based on evolutionary concepts.

Abstract

The tenacity by which barnacles adhere has sparked a long history of scientific investigation into their adhesive mechanisms. To adhere, barnacles utilize proteinaceous cement that rapidly polymerizes and forms adhesive bonds underwater, and is insoluble once polymerized. Although progress has been made towards understanding the chemical properties of cement proteins, the biochemical mechanisms of cement polymerization remain largely unknown. In this dissertation, I used evolutionary concepts to elucidate barnacle cement polymerization. Well-studied biological phenomena (blood coagulation in vertebrates and invertebrates) were used as models to generate hypotheses on proteins/biochemical mechanisms involved in cement polymerization. These model systems are under similar selective pressures to cement polymerization (life or death situations) and show similar chemical characteristics (soluble protein that quickly/efficiently coagulates). I describe a novel method for collection of unpolymerized cement. Multiple, independent techniques (AFM, FTIR, chemical staining for peroxidase and tandem mass spectroscopy) support the validity of the collection technique. Identification of a large number of proteins besides ‘barnacle cement proteins’ with mass spectrometry, andobservations of hemocytes in unpolymerized cement inspired the hypothesis that barnacle cement is hemolymph. A striking biochemical resemblance was shown between barnacle cement polymerization and vertebrate blood coagulation. Clotted fibrin and polymerized cement were shown to be structurally similar (mesh of fibrous protein) but biochemically distinct. Heparin, trypsin inhibitor and Ca2+ chelators impeded cement polymerization, suggesting trypsin and Ca2+ involvement in polymerization. The presence/activity of a cement trypsin-like serine protease was verified and shown homologous to bovine pancreatic trypsin. Protease activity may activate cement structural precursors, allowing loose assembly with other structural proteins and surface rearrangement. Tandem mass spectrometry and Western blotting revealed a homologous protein to human coagulation factor XIII (fibrin stabilizing factor: transglutaminase that covalently cross-links fibrin monomers). Transglutaminase activity was verified and may covalently cross-link assembled cement monomers. Similar to other protein coagulation systems, heritable defects occur during cement polymerization. High plasma protein concentration combined with sub-optimal enzyme, and/or cofactor concentrations and sub-optimal physical/muscular parameters (associated with hemolymph release) results in improperly cured cement in certain individuals when polymerization occurs in contact with low surface energy silicone and its associated leached molecules.